This invention relates to glass manufacture, and in particular, to preheating of cullet and glass batch utilized as a feedstock in a glass furnace.
Glass manufacturing is an energy-intensive process which requires about five-million Btu/ton for the melting process alone. As of the early 1980's, 300,000 billion Btu were consumed annually by the glass industry in the United States, with natural gas providing 70% of this energy. In a typical glass furnace, the heat supplied to the melt is provided predominantly by natural gas mixed with preheated combustion air. The resultant flame fires over the melt and heat is transferred to the melt by radiation from the flame and furnace enclosure.
Additionally, some furnaces augment the heat with an electric boost. The major attraction of a electric boost is that it allows an increase in production from an existing furnace by providing an additional heat source to the melt. Typically, for a container furnace (a glass furnace for manufacturing bottles), an equivalent of 1200 kW or more can be added directly into the glass furnace by boosting, which yields a production increase of 20%.
In addition to energy costs, another significant factor in glass manufacturing is the cost of the constituents which are melted to form the glass. Traditionally, glass batch composed of compounds including Na.sub.2 Co.sub.3, CaCo.sub.3, and SiO.sub.2 is used to form glass. In recent years, the glass industry has been able to reduce this part of the cost by supplementing the traditional glass batch constituents with recycled glass, commonly referred to as cullet. The cullet is obtained from sources such as recycled bottles, which are broken up into practical sizes for use as glass forming constituents.
At present, the maximum amount of glass that can be pulled from a furnace is limited by the amount of energy that can be put into the furnace to melt the feedstock. This limit is reached when the furnace is at high fire resulting in maximum flow of the products of combustion.
A method of increasing the amount of heat that can be added to a furnace while avoiding the high operating cost of an electric boost is to preheat the feedstock- i.e. the glass batch and the cullet. Preheating the batch and the cullet separately has proven feasible. However, there is a need for a preheater that can preheat both the cullet and the glass batch together.
As indicated above, additional energy can be put into the furnace by preheating the glass batch and cullet so as to provide a more productive and efficient glass manufacturing process. One suggested method of preheating the cullet and glass batch is that of a fluidized bed preheater. A problem with this type of preheater is that the glass forming mixture contains large particles with a wide size distribution. The high cost to grind these large particles to a consistent, workable distribution for use in a fluidized bed makes this process unattractive.
Another method of preheating the starting materials is taught in U.S. Pat. No. 4,374,660, Fluidized Bed Glass Batch Preheater, issued to Sakhuja et. al.. The Sakhuja patent teaches a system for heating starting materials for glass melting and means for separating the lighter particles entrained in the hot gases and reintroducing them to the heated starting materials. The Sakhuja patent has the disadvantage of requiring two different heating systems the cullet is preheated using a packed bed and the glass batch is preheated with a fluid bed arrangement. Also, as mentioned above, the cullet could not be combined with the glass batch into the fluid bed without grinding the cullet beforehand.
Additionally, combining the cullet and the glass batch in the packed bed has the problems of any packed bed system. A substantial disadvantage of packed bed heating is that the heating gas tends to carve out a path for itself through the bed of material resulting in an uneven temperature distribution. Also, the high temperature drop across a packed bed would require the use of a substantial amount of auxiliary power to direct gases through the materials. In addition, a very high residence time is required to heat the materials to the desired temperature.
U.S. Pat. No. 4,875,919, entitled "Direct Contact Raining Bed Counterflow Cullet Preheater and Method for Using Same", by Desaro et. al., whose disclosure is incorporated herein by this reference to that patent, describes a system which can efficiently preheat the larger particles in the feedstock, typically the cullet, without the disadvantages of either packed or fluid bed preheater systems. However, the system described in this patent cannot preheat the combined feedstock without preparing the glass batch into pellets.
Without this preparation, the fine particles in the feedstock, typically glass batch, become entrained in the hot gas and flow out the top of the preheater. Thus, some of the glass batch is the wasted and the glass batch that does get preheated may be of the wrong composition to form glass. A separate preheater could be used for the glass batch but this would be less desirable than a single preheater
In short, the glass industry is in need of a simple, cost-effective system, by which it can increase the productivity of furnaces by preheating cullet and glass batch feedstock together prior to the melting process.
Therefore, it is an object of the present invention to provide a means by which existing glass furnaces can be made more productive and energy efficient.
A further object of the present invention is to overcome known production limitations of glass furnaces by delivering more energy to the furnace through glass forming mixture preheating.
Another object of the invention is to provide an improved method and apparatus for preheating the glass forming mixture, cullet and glass batch feedstock, for use in a glass furnace.